This invention relates to an electronic circuit package and a process of producing the same, and more particularly to an electronic circuit package suitable for use with ICs for very high speed optical communications, which is used in an optical repeater or the like and a process of producing such electronic circuit package.
In recent years, very wide band single mode optical fibers have been put into practical use as optical transmission lines, and the application range of high speed transmission systems is being expanded widely. Nowadays, a 1.6 Gb/s system is introduced into a main line system. Meanwhile, as the service to subscribers is expanded, the tendency of increasing the speed of a transmission system is expanding progressively, and the speed target of a next term system is raised at a stroke to 10 Gb/s in connection with construction of a network based on a new synchronous network. In order to realize a system which deals with electric signals in a so-called microwave region, not only it is necessary to develop optical elements and electronic circuits which can operate in a microwave region, but also it is necessary to develop electronic circuit packages for packaging such elements and electronic circuits without deteriorating high speed characteristics of them.
An exemplary one of known packages for ICs (integrated circuits) in a system having a transmission speed of up to several Gb/s which have been put into practical use in a main line system has a layer structure of an insulating substance wherein a conductor pattern is interposed for each layer and electric connection between different layers is established by way of through-holes formed in the layers. The electronic package, however, cannot sufficiently satisfy such demands as listed below and accordingly cannot be applied readily to a very high speed system having a transmission speed higher than several Gb/s.
(1) Strengthening of Grounding
As the speed of a signal rises, it becomes a technical subject whether or not grounding of an electronic circuit can be assured sufficiently. For example, connection between a ground port of an IC chip and a grounding portion of an external circuit is normally established by means of wire bonding, but in a high frequency region, such bonding wire will act as an inductance, which will make grounding insufficient. Meanwhile, if the external grounding portion itself is produced by drawing of a conductor pattern or the like, then the conductor pattern or the like itself may have an impedance. Where grounding is not sufficient, a common potential with respect to input signals from the outside will not be present, which will have such a significant bad influence that crosstalk or noises are increased or an input reflection characteristic is deteriorated. Further, a circuit construction of a differential couple such as an ECL (Emitter Coupled Logic) or an SCFL (Source Coupled Logic) which is used principally as a high speed circuit is essentially tough against a variation of a power source, but is weak for a variation of a ground potential, and a stable operation cannot be assured. Accordingly, it is demanded to strengthen grounding to restrain deterioration of a high speed characteristic.
(2) Decrease of External Junctions of High Speed Signals
As the speed of a signal rises, the difficulty in interface between IC chips increases, and in order to assure required characteristics, it is necessary to establish connection between chips by way of a transmission line which requires impedance matching. A matched impedance is suitable at 50 .OMEGA. for the facility in measurement. Meanwhile, as the speed of a signal rises, the difficulty in drawing of a transmission line increases, and besides a high speed characteristic may be deteriorated significantly depending upon a condition of wire bonding, soldering and so forth. Accordingly, it is desirable to decrease treatment of high speed signals outside an IC chip as little as possible. Consequently, it is demanded to decrease interface points to high speed signals outside an IC chip or outside a package as little as possible to restrain deterioration of a high speed characteristic.
(3) Matching of High Speed Signal Lines
As described above, a high speed signal should be transmitted over an impedance line of 50 .OMEGA.. In this instance, it is demanded for the impedance line and terminal conditions to assure matching over an entire applicable frequency range. For example, although it may overlap with the demand of (1) above, if grounding of a ground pattern extending in parallel to a signal strip line on a circuit board is not perfect even in a high frequency, the strip line will not exhibit required characteristics as a transmission line and will be deteriorated in input reflection characteristic. Accordingly, it is demanded to assure matching of high speed signal lines to restrain deterioration of a high speed characteristic.
(4) Prevention of Crosstalk
There is the possibility that a high speed characteristic may be deteriorated or oscillation may be caused by crosstalk between an input and an output of an IC chip or crosstalk between chips in the case of multichip packaging. Particularly in the field of optical communications, countermeasures to crosstalk are important for the object of improvement in function, increase in gain and promotion in high integration of an IC chip and increase in scale and aggregation in function of a package. Further, in the case of a circuit board having a large size, the circuit board itself may act as a cavity resonator, or in the case of a cavity in which an IC chip is accommodated, the spacing itself of the cavity may act as a cavity resonator, which will remarkably deteriorate a crosstalk characteristic at its resonance frequencies. Therefore, it is also necessary to take measures against such deterioration in crosstalk characteristic. Accordingly, it is demanded to prevent crosstalk to restrain deterioration of a high speed characteristic.
(5) Countermeasures for Heat Radiation
By the way, if the integration of an IC chip is improved or the speed of an electronic circuit is raised in form or else a terminal resistor is built in an IC chip, then the power consumption of an IC chip will be increased, that is, the amount of heat generation will be increased. Meanwhile, where an IC chip is of the silicon type, the substrate thereof cannot always be held at a ground potential because it is necessary to keep the rear face side (substrate) of the IC chip at a lowest potential. Accordingly, in such an instance, the IC chip cannot be securely mounted directly on a grounding portion, and countermeasures for heat radiation are particularly important. Accordingly, it is demanded to take measures for both heat radiation and restraint of a high speed characteristic deterioration.